In EUV lithography, highly integrated structures with a line width of less than 50 nm are produced by microlithographic projection devices. Use is here made of laser radiation from the EUV range (extreme ultraviolet light, also called soft X-ray radiation) with wavelengths of about 13 nm. The projection devices are equipped with mirror elements, which consist of titanium dioxide-doped glass having a high silicic acid content (hereinafter also referred to as “Ti-doped silica glass”) and are provided with a reflective layer system. These materials are distinguished by an extremely low coefficient of thermal expansion, so that they do not deform due to heating during the exposure process, which would lead to a deterioration of the imaging quality.
Ti-doped silica glass is produced by flame hydrolysis, starting from silicon- and titanium-containing precursor substances. A porous soot body of titanium-doped SiO2 is first produced and is vitrified into a dense glass body after drying for reducing the hydroxyl group content (OH content). However, due to a more or less strong concentration of Ti3+ ions in the glass matrix, the doping process with titanium oxide results in a brownish coloration of the glass. The moldings for this application, hereinafter also called blanks, are large, dark-brown plates with dimensions of up to about 70×60×20 cm3, which must be checked for their optical properties and/or for defects caused by the manufacturing process or for inhomogeneities. The brownish coloration of the glass has been found to be problematic because optical measurement methods that require transparency in the visible spectral range can thus be used only to a limited degree or not at all.
The literature has proposed various solutions for restricting the amount of the Ti3+ ions in favor of Ti4+ ions by way of an oxidation treatment. When a Ti-doped silica glass with a relatively high hydroxyl group content is used, the OH groups permit the desired oxidation of Ti3+ into Ti4+. This is described by Carson and Maurer in “Optical Attenuation in Titania-Silica Glasses”, J. Non-Crystalline Solids, Vol. 11(1973), pp. 368-380, wherein a reaction according to formula 2Ti3++2OH−→2Ti4++2O2−+H2 is indicated. In a corresponding thermal treatment, the OH groups contained in the Ti-doped soot body oxidize the Ti3+ ions into Ti4+ ions, with the resulting hydrogen diffusing out of the porous soot body.
This procedure is adopted in EP 2 428 488 A1, particularly with a view to optimized conditions for the process regarding oxidation and out-diffusion of the hydrogen. The Ti-doped silica glass disclosed in EP 2 428 488 A1 has a high OH content of more than 600 wt. ppm and a relatively low hydrogen content (less than 2×1017 molecules/cm3). To ensure a high OH content, it is recommended that water vapor should be added during deposition. The amount of Ti3+ ions is indicated to be less than 3 ppm.
WO 2004/089836 A1 discloses a Ti-doped silica glass with fluorine codoping that shows a very flat curve of the thermal expansion coefficient over a relatively wide temperature range. First of all, the porous TiO2—SiO2 soot body is pre-dried in air at 1200° C.; this entails a first reduction of the OH content and an oxidation of Ti3+ ions. Subsequently, for fluorine doping the TiO2—SiO2 soot body is exposed to an atmosphere with 10% by vol. of SiF4 at 1000° C. for several hours. Apart from fluorine doping, this treatment also entails a further reduction of the OH content. To prevent any dark coloration during vitrification of the soot body, it is suggested according to WO 2004/089836 A1 that the soot body should be treated in an oxygen atmosphere in the temperature range between 300° C. and 1300° C. for several hours before the vitrification step is subsequently carried out.
Similarly, WO2006/004169 A1 also suggests an oxygen treatment of a TiO2—SiO2 soot body with F codoping prior to vitrification so as to prevent any dark coloration due to reduction of TiO2. The Ti-doped silica glass produced in this way contains 10 wt. ppm OH groups and 12 wt. ppm Ti3+ ions at a fluorine content of 6300 wt. ppm.
WO2009/128560 A1 starts from a SnO2—TiO2—SiO2 glass which, like the Ti-doped silica glass, is also distinguished by a thermal expansion coefficient of zero. Tin and also other elements mentioned in WO2009/128560 A1 act as a so-called Ti3+ inhibitor. A precondition is, however, that the tin is at least predominantly present as Sn4+ ion or as SnO2. That is, when a relevant amount of Sn2+ is present, the desired effect with respect to a reduction of the concentration of Ti3+ ions cannot be observed. Rather, Sn2+ additionally contributes to the absorption in the visible wavelength range. On the other hand, an excessive amount of Sn4+ ions entails the risk of crystalline SnO2 depositions. All in all, the SnO2—TiO2—SiO2 glass must therefore be set very accurately in its composition, which makes its production complicated. WO2009/128560 A1 therefore suggests an oxygen treatment of the corresponding soot body prior to vitrification also for this glass.
In summary, it should be noted that according to the prior art, the reduction of Ti3+ ions in favor of Ti4+ ions in Ti-doped silica glass is ensured by a sufficiently large amount of OH groups, whereby an internal oxidation takes place with out-diffusion of hydrogen, or at a low OH group content an oxygen treatment is needed prior to vitrification, which oxygen treatment requires a high treatment temperature and special corrosion-resistant furnaces and is thus expensive.